Battery monitoring system

ABSTRACT

In a battery monitoring system including a plurality of battery modules each including one or more cells, the battery modules are connected in series to each other. The battery monitoring system monitors the state of each cell based on the voltage value of the cell and the current value of the battery modules. A current detection unit detects the current value. Each voltage detection unit is associated with the corresponding one of the battery modules and detects the voltage value. Each slave unit is associated with the corresponding one of the battery modules, and wirelessly transmits information including synchronous current and voltage values detected by the current detection unit and the voltage detection unit. A master unit receives the information transmitted from the slave units. A central monitoring unit receives the information received by the master unit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2018-179509 filed Sep. 25, 2018, the description of which is incorporated herein by reference.

BACKGROUND Technical Field

This disclosure relates to a battery monitoring system for monitoring a plurality of battery modules each including a battery cell.

Related Art

A system is known that includes different battery cell managers provided for different battery modules, and a single battery manager serving the multiple battery cell managers. Each battery cell manager detects the voltage value of each battery cell included in the battery module associated with the battery cell manager, and wirelessly transmits the detected voltage value. The battery manager receives the voltage value wirelessly transmitted from each battery cell manager, and calculates the state of charge of the battery cell from the received voltage value and the current value of the battery cell, which is received in a different way.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram showing a battery monitoring system according to a first embodiment;

FIG. 2 is an enlarged view of a part of FIG. 1;

FIG. 3 is a flowchart showing the operation of the battery monitoring system;

FIGS. 4A to 4E are timing charts showing times to acquire voltage data and current data;

FIGS. 5A to 5F are timing charts showing times to acquire voltage data and current data in a modification;

FIG. 6 is a schematic diagram showing a battery monitoring system according to a second embodiment;

FIG. 7 is a schematic diagram showing a battery monitoring system according to a third embodiment;

FIGS. 8A to 8E are graphs showing current data transmission;

FIG. 9 is a schematic diagram showing a battery monitoring system according to a fourth embodiment;

FIG. 10 is a schematic diagram showing a battery monitoring system according to a fifth embodiment;

FIG. 11 is a schematic diagram showing a battery monitoring system according to a sixth embodiment;

FIG. 12 is a schematic diagram showing a battery monitoring system according to a seventh embodiment; and

FIGS. 13A to 13C are graphs and a formula for describing a method of calculating a current value.

DESCRIPTION OF SPECIFIC EMBODIMENT

The known system, as disclosed in Japanese Patent No. 6093448, can have a smaller wiring space because each battery cell manager wirelessly transmits a voltage value. This contributes to space saving. In addition, even if an attempt at wireless communication fails under one condition, the system can retry the wireless communication under another condition. For example, the system can retry the wireless communication in a different frequency band or via a different wireless module to establish the wireless communication. However, retrying wireless communication may cause a difference between a voltage value detection time and a current value detection time. As a result, the voltage value and the current value may become out of synchronization. The failure of synchronization reduces the accuracy in the battery manager calculating the state of each battery cell, resulting in a rapid deterioration of the battery cell.

In view of the foregoing, it is desired to have a battery monitoring system that occupies less space saved and ensures the accuracy in calculation of the state of each battery cell.

Embodiments of the present disclosure will now be described with reference to the drawings. However, the present disclosure is not limited to the embodiments. The disclosure may be modified as appropriate without departing from the gist of the disclosure.

The terms “synchronous” and “substantially concurrent” as used herein mean that the difference between voltage value and current value detection times falls within a range that does not affect the accuracy in calculation of the internal resistance of each cell.

First Embodiment

FIG. 1 is a schematic diagram showing a battery monitoring system 90 according to the present embodiment. The battery monitoring system 90 monitors a plurality of battery modules 80. Each battery module 80 includes a plurality of cells 81 connected in series to each other through a series circuit 85. The battery modules 80 are also connected in series to each other through the series circuit 85. The battery monitoring system 90 includes a current sensor 10, a plurality of satellite units 20, and a battery electronic control unit (ECU) 30. The current sensor 10 and the satellite units 20 are cable-connected to each other via a cable 41 in a communicable manner. The cable 41 further connects the current sensor 10 and the battery ECU 30 in a communicable manner.

FIG. 2 is an enlarged view of a part of FIG. 1. In the present embodiment, the current sensor 10 corresponds to a current detection unit in the present disclosure. The single current sensor 10 serves the multiple satellite units 20. The current sensor 10 is connected to a power supply 43 via a line 42 and supplied with appropriate power from the power supply 43. The current sensor 10 includes a current detection circuit 11 and a digital communication unit 12. The current detection circuit 11 detects the value of a current flowing in the series circuit 85. The detection may be achieved by detecting the magnetic field intensity corresponding to the current value, in a state electrically isolated from the series circuit 85. The value of a current flowing in the series circuit 85 corresponds to the value of a current flowing in each battery module 80 and its cells 81. The digital communication unit 12 converts the current value detected by the current detection circuit 11 into a digital signal, and outputs the resultant digital signal. In the present embodiment, the current sensor 10 successively detects current values and outputs the values in the form of digital current data, or the signals representing the values. However, the current sensor 10 may detect a current value in response to a current acquisition request and output the value in the form of digital current data. In this case, the digital communication unit 12 receives a current acquisition request from a satellite unit 20.

Each satellite unit 20 is associated with the corresponding one of the battery modules 80. The satellite unit 20 includes a first isolator 21, a monitoring IC 22, a second isolator 23, and a slave unit 24. In the present embodiment, the monitoring IC 22 corresponds to a voltage detection unit in the present disclosure.

The digital communication unit 12 and each monitoring IC 22 are cable-connected to each other through the cable 41 and the first isolator 21 in a communicable manner. The first isolator 21 may be a magnetic coupler or a photocoupler. The first isolator 21 is installed to electrically isolate the digital communication unit 12, which has a low-voltage circuit, from the monitoring IC 22, which has a high-voltage circuit, and transfer current data.

The monitoring IC 22 receives current data via cable from the digital communication unit 12 as a digital signal through the first isolator 21. The monitoring IC 22 includes a voltage detection circuit, a register, and a control unit. The voltage detection circuit detects the voltage value (terminal voltage) of each cell 81 in the battery module 80. The register stores voltage data generated by converting a voltage value detected by the voltage detection circuit into a signal, current data received from the current sensor 10 via cable, and other data. The control unit controls the voltage detection circuit and the register.

The monitoring IC 22 and the slave unit 24 are cable-connected to each other through the second isolator 23 in a communicable manner. The second isolator 23 may be a magnetic coupler or a photocoupler. The second isolator 23 is installed to electrically isolate the monitoring IC 22, which has the high-voltage circuit, from the slave unit 24, which has a low-voltage circuit, and transfer voltage data and current data.

The slave unit 24 receives current data and voltage data in the form of digital signals from the monitoring IC 22 via cable through the second isolator 23. The slave unit 24 has an antenna 24 a and causes the antenna 24 a to wirelessly transmit the current data and the voltage data received via cable. The slave unit 24 also causes the antenna 24 a to wirelessly receive an instruction from the battery ECU 30.

The battery ECU 30 includes a master unit 31, a micro controller unit (MCU) 32, and a power unit 33. In the present embodiment, the MCU 32 corresponds to a central monitoring unit in the present disclosure.

The master unit 31 has an antenna 31 a and causes the antenna 31 a to wirelessly receive current data and voltage data wirelessly transmitted from the slave units 24. The master unit 31 also causes the antenna 31 a to wirelessly transmit an instruction to a satellite unit 20. The master unit 31 and the MCU 32 are cable-connected to each other in a communicable manner.

The MCU 32 uses voltage data and current data received through a slave unit 24 and the master unit 31 to calculate the internal resistance of the cells 81. Based on the internal resistance, the MCU 32 calculates the state of charge (SOC) of the cells 81. The MCU 32 also receives current data from the digital communication unit 12 through the cable 41, and calculates the sum of the current in the cells 81 from the current data.

The power unit 33 is connected to the power supply 43 via the line 42 and sends the power supplied from the power supply 43 to the MCU 32 and the master unit 31.

FIG. 3 is a flowchart showing the operation of the battery ECU 30 and the satellite units 20. The battery ECU 30 causes the MCU 32 to generate a voltage acquisition request (S301), and wirelessly transmits the voltage acquisition request from the master unit 31 (S302). Then, the battery ECU 30 determines whether the wireless transmission has been received by each slave unit 24, or in other words, whether wireless communication has been established (S303). If no establishment is identified (S303: NO), the battery ECU 30 retries the wireless communication.

If the wireless communication is established (S303: YES), the satellite unit 20 acquires the voltage data on the cells 81 through the voltage detection circuit of the monitoring IC 22, and also acquires the synchronous current data from the current sensor 10 (S204). The voltage data and current data are stored in the register of the monitoring IC 22. The satellite unit 20 wirelessly transmits the acquired current data and voltage data from the slave unit 24 (S205). Then, the satellite unit 20 determines whether the wireless transmission has been received by the master unit 31, or in other words, whether wireless communication has been established (S206). If no establishment is identified (S206: NO), the satellite unit 20 retries the wireless communication.

If the wireless communication is established (S206: YES), the battery ECU 30 causes the MCU 32 to calculate the internal resistance of the cells 81 from the voltage data and the current data, and also calculate the state of charge of the cells 81 based on the internal resistance (S307).

FIGS. 4A to 4E are timing charts showing when the register acquires voltage data and current data in the present embodiment, in which the current sensor 10 successively outputs current data. More specifically, FIG. 4A is a timing chart showing turning on and off of a voltage acquisition request by the MCU 32. FIG. 4B is a timing chart showing output of current data by the digital communication unit 12. FIG. 4C is a timing chart showing turning on and off of a voltage acquisition flag by the control unit of the monitoring IC 22. FIG. 4D is a timing chart showing output of voltage data by the voltage detection circuit of the monitoring IC 22. FIG. 4E is a timing chart showing storing the voltage data and the current data in the register of the monitoring IC 22.

As shown in FIG. 4A, when the MCU 32 issues a voltage acquisition request (S301), the voltage acquisition request passes through the master unit 31 and the slave unit 24 (S302, S303), reaching the monitoring IC 22. As shown in FIG. 4C, the control unit of the monitoring IC 22 turns the voltage acquisition flag on. As a result, the voltage detection circuit of the monitoring IC 22 detects the voltage value of each cell 81 and outputs the corresponding voltage data, as shown in FIG. 4D. The voltage data and the synchronous current data in FIG. 4B output from the current sensor 10 are stored in the register of the monitoring IC 22 (S204), as shown in FIG. 4E. At this time, the control unit of the monitoring IC 22 determines an acquisition range of digital signals successively output from the current sensor 10 to set one frame of current data.

FIGS. 5A to 5F are graphs showing when the register acquires voltage data and current data in a modification of the present embodiment, in which the current sensor 10 detects a current value to output current data in response to a current acquisition request. More specifically, FIGS. 5A and 5B are timing charts corresponding to FIGS. 4A and 4B. FIG. 5C is a timing chart showing turning on and off of a current acquisition flag by the control unit of the monitoring IC 22. FIGS. 5D to 5F are timing charts corresponding to FIGS. 4C to 4E.

As shown in FIG. 5A, when the MCU 32 issues a current acquisition request and a voltage acquisition request (S301), the current acquisition request and the voltage acquisition request pass through the master unit 31 and the slave unit 24 (S302, S303), reaching the monitoring IC 22. As shown in FIG. 5C, the control unit of the monitoring IC 22 turns the current acquisition flag on, and as shown in FIG. 5D, turns the voltage acquisition flag on. As a result, the current sensor 10 detects the current value and outputs the corresponding current data as shown in FIG. 5B, while the voltage detection circuit of the monitoring IC 22 detects the voltage value of each cell 81 and outputs the corresponding voltage data as shown in FIG. 5E. The voltage data and the current data are stored in the register of the monitoring IC 22, as shown in FIG. 5F (S204).

According to the present embodiment, in which each slave unit 24 wirelessly transmits data to the master unit 31, the wiring space between the slave unit 24 and the master unit 31 can be reduced compared with a system that transmits data via cable. The slave unit 24, which receives voltage data and current data via cable, does not involve retrying failed wireless communication, unlike a unit that wirelessly receives such data. Accordingly, such a retry would not cause a difference between a voltage value detection time and a current value detection time. As a result, the voltage value and the current value can be synchronized. After that, even if wireless transmission fails and the wireless transmit is retried, the voltage value detection time and the current value detection time will not become out of synchronization. This enables the MCU 32 to calculate the internal resistance of each cell 81 with improved accuracy, thus enhancing the accuracy in calculation of the state of charge of the cells 81.

The use of the single current sensor 10 for the multiple satellite units 20 can simplify the structure of the battery monitoring system 90 and also reduce costs.

The monitoring IC 22 can integrate a current value and a voltage value because the monitoring IC 22 receives current data from the current sensor 10 via cable and transmits the current data and the voltage data on a detected voltage value to the slave unit 24 via cable. The series cable-connection between the current sensor 10 and the monitoring IC 22 simplifies the structure of the satellite unit 20.

The isolators 21 and 23, which transfer digital signals while providing isolation, permit a potential difference between the current sensor 10 and the slave unit 24, such as a voltage higher in the monitoring IC 22 than in the current sensor 10 or a voltage lower in the slave unit 24 than in the monitoring IC 22, as in the present embodiment.

Second Embodiment

A second embodiment of the present disclosure will now be described. In the present embodiment, components identical or similar to those in the first embodiment are designated by the same reference numerals, and differences from the first embodiment will be mainly described.

FIG. 6 is a schematic diagram showing a battery monitoring system 90 according to the present embodiment. The current sensor 10 is cable-connected not to the monitoring IC 22 but to the slave unit 24 in a communicable manner. The first isolator 21 is installed between the current sensor 10 and the slave unit 24. The slave unit 24 thus receives voltage data from the monitoring IC 22 via cable, while receiving current data from the current sensor 10 via cable, not through the monitoring IC 22. The voltage data and the current data substantially concurrently detected by the current sensor 10 are stored in the register of the slave unit 24.

In the present embodiment, the slave unit 24 can integrate a current value and a voltage value. Furthermore, current data detected by the current sensor 10 can be directly transmitted to the slave unit 24 via cable, not through the monitoring IC 22. This makes it easy to wirelessly transmit voltage data in synchronization with current data in response to a voltage acquisition request, while successively transmitting current data to the battery ECU 30. This allows the MCU 32 to easily presume the current value synchronized with a voltage value by comparing the current values representing current data regularly received through the slave unit 24 and the master unit 31, with the current values representing current data received through the cable 41.

Third Embodiment

A third embodiment of the present disclosure will now be described. In the present embodiment, components identical or similar to those in the first embodiment are designated by the same reference numerals, and differences from the first embodiment will be mainly described.

FIG. 7 is a schematic diagram showing a battery monitoring system 90 according to the present embodiment. The cable 41 connects the current sensor 10 not to the battery ECU 30 but to each satellite unit 20. In other words, the battery monitoring system 90 has no path for carrying current data via cable from the current sensor 10 to the MCU 32. The MCU 32 thus calculates the sum of the current in the cells 81 based on current data wirelessly transmitted from different slave units 24 at different times.

FIGS. 8A to 8E are graphs showing current data transmission and acquisition. More specifically, FIG. 8A is a graph showing an actual current flowing in the cells 81. FIG. 8B is a graph showing current values acquired by the battery ECU 30. FIGS. 8C to 8E are graphs showing current values acquired by different satellite units 20. Each satellite unit 20 acquires current data more frequently than voltage data, and wirelessly transmits the acquired current data to the battery ECU 30. Each satellite unit 20 and the other satellite units 20 acquire current data at different times, and wirelessly transmit the acquired current data to the battery ECU 30. The battery ECU 30 can receive current data acquired by the different satellite units 20 at different times, thus determining fluctuations in current values as shown in FIG. 8B.

In the present embodiment, the removal of the cable between the current sensor 10 and the MCU 32 can simplify the structure of the battery monitoring system 90 and also reduce costs. The MCU 32 can receive current data relatively frequently without being cable-connected to the current sensor 10 because each slave unit 24 wirelessly transmits current data more frequently than voltage data. The MCU 32 can receive current data relatively frequently without being cable-connected to the current sensor 10 also because each slave unit 24 and the other slave units 24 wirelessly transmit current data at different times. The MCU 32 can thus receive current data quite frequently. The received current data can be used to calculate the sum of the current in the cells 81.

Fourth Embodiment

A fourth embodiment of the present disclosure will now be described. In the present embodiment, components identical or similar to those in the first embodiment are designated by the same reference numerals, and differences from the first embodiment will be mainly described.

FIG. 9 is a schematic diagram showing a battery monitoring system 90 according to the present embodiment. The monitoring IC 22 and the slave unit 24 have the same voltage. Thus, no second isolator 23 is installed between the monitoring IC 22 and the slave unit 24, with the monitoring IC 22 and the slave unit 24 directly cable-connected to each other. In the present embodiment, the second isolator 23 is not required, which allows the area of the substrate to be reduced, thus contributing to the reduction of costs.

Fifth Embodiment

A fifth embodiment of the present disclosure will now be described. In the present embodiment, components identical or similar to those in the first embodiment are designated by the same reference numerals, and differences from the first embodiment will be mainly described.

FIG. 10 is a schematic diagram showing a battery monitoring system 90 according to the present embodiment. The current sensor 10 is installed in each satellite unit 20, or more particularly, provided for each slave unit 24. Each slave unit 24 receives current data via cable from the corresponding current sensor 10. More specifically, the monitoring IC 22 detects and turns voltage values into voltage data, and receives the substantially concurrently detected current data from the current sensor 10 via cable. The monitoring IC 22 transmits the voltage data and the current data to the slave unit 24 via cable.

In the present embodiment, the collection of voltage data and current data is completed within each satellite unit 20 because the current sensor 10 is installed in each satellite unit. This enables voltage values and current values to be easily synchronized with each other.

Sixth Embodiment

A sixth embodiment of the present disclosure will now be described. In the present embodiment, components identical or similar to those in the first and the fifth embodiments are designated by the same reference numerals, and differences from the fifth embodiment will be mainly described.

FIG. 11 is a schematic diagram showing a battery monitoring system 90 according to the present embodiment. The battery monitoring system 90 in the present embodiment includes a current detection unit 16 in place of the current sensor 10. The current detection unit 16 includes a shunt resistor 16 a, an amplifier circuit 16 b, and a calculator. The shunt resistor 16 a is connected in series to the battery modules 80. The amplifier circuit 16 b amplifies the voltage difference across the shunt resistor 16 a, as well as converts the amplified voltage signal into a digital signal and transmits the resultant signal to the calculator (the monitoring IC 22) via cable. The calculator is installed in the monitoring IC 22 and calculates a current value from the received digital signal. The accuracy in the current value detection can be enhanced by improving the calculation accuracy at the calculator. In the present embodiment, the non-necessity of the current sensor 10 can contribute to the reduction of costs.

Seventh Embodiment

A seventh embodiment of the present disclosure will now be described. In the present embodiment, components identical or similar to those in the first and the fifth embodiments are designated by the same reference numerals, and differences from the fifth embodiment will be mainly described.

FIG. 12 is a schematic diagram showing a battery monitoring system 90 according to the present embodiment. The battery monitoring system 90 in the present embodiment includes a current detection unit 17 in place of the current sensor 10 to acquire the voltage of the battery module 80 by the flying capacitor method. The current detection unit 17 includes a first switch 17 a, a capacitor 17 b, a second switch 17 c, a voltage sensor 17 d, and a calculator 17 e. The calculator 17 e is cable-connected to the monitoring IC 22 in a communicable manner. When the first switch 17 a is closed, the capacitor 17 b is connected in parallel with the battery module 80. When the first switch 17 a is open, the parallel connection is released. When the second switch 17 c is closed, the voltage sensor 17 d is connected in parallel with the capacitor 17 b. When the second switch 17 c is open, the parallel connection is released.

The process of current value detection by the current detection unit 17 in the present embodiment will now be described. First, the first switch 17 a and the second switch 17 c are both open. In this state, the first switch 17 a is closed for a predetermined period of time t to charge the capacitor 17 b, and then the first switch 17 a is opened again. In this state, the second switch 17 c is then closed to detect the voltage V of the charged capacitor 17 b. The capacitance of the capacitor 17 b is denoted by C.

The calculator 17 e then calculates a current value from the values t, C, and V. More specifically, with a current value denoted by i, the current value is calculated in accordance with the formula, i=CV/t. The accuracy in the current value detection can be enhanced by improving the calculation accuracy at the calculator 17 e. FIG. 13A is a graph showing an actual current flowing in the cells 81. FIG. 13B is a graph showing voltages acquired by the capacitor 17 b. FIG. 13C is the formula for calculating a current value.

The calculation result (current value) obtained at the calculator 17 e is converted into a digital signal and then transmitted to the monitoring IC 22 via cable. In the present embodiment, the non-necessity of the current sensor 10 can contribute to the reduction of costs.

Other Embodiments

The embodiments may be modified as described below. For example, instead of wirelessly transmitting current data and voltage data from each slave unit 24 to the master unit 31, each satellite unit 20, or more specifically, the monitoring IC 22, the slave unit 24, and other calculators may calculate the internal resistance and the state of charge of the cells 81 from the current data and the voltage data, and wirelessly transmit the calculation result data from the slave unit 24 to the master unit 31.

In an example, instead of connecting the battery ECU 30 and the current sensor 10 via cable, the battery ECU 30 and the current sensor 10 may wirelessly communicate with each other. In another example, voltage data and current data may be transmitted not in a digital form but in an analog form. In yet another example, the voltage of the digital communication unit 12 may be equalized with the voltage of the monitoring IC 22 to remove the first isolator 21. In still another example, the calculator in the sixth embodiment may be installed not in the monitoring IC 22 but in the slave unit 24.

In an example, the current sensor 10 in the first to the fourth embodiments may be replaced with the current detection unit 16 in the sixth embodiment or the current detection unit 17 in the seventh embodiment. The current detection unit 16 or 17 may detect and transmit current data to each satellite unit 20 via cable. In another example, without installing the digital communication unit 12 in the current sensor 10 in the first to the fifth embodiments, output from the current sensor 10 may be digitized in the monitoring IC 22 or the slave unit 24. In yet another example, output from the current detection unit 16 or 17 in the sixth or the seventh embodiment may be digitized in the monitoring IC 22 or the slave unit 24.

In an example, the slave unit 24 in any embodiment other than the second embodiment may directly receive current data from the current sensor 10 via cable, not through the monitoring IC 22. In another example, each satellite unit 20 in any embodiment other than the second and the third embodiments may also acquire current data more frequently than voltage data and wirelessly transmit the current data to the battery ECU 30. 

What is claimed is:
 1. A battery monitoring system for monitoring a plurality of battery modules each including one or more cells, the battery modules being connected in series to each other, and the battery monitoring system being configured to monitor a state of each cell based on a voltage value of the cell and a current value of the battery modules, the battery monitoring system comprising: at least one current detection unit configured to detect the current value; voltage detection units each associated with a corresponding one of the battery modules and configured to detect the voltage value; slave units each associated with a corresponding one of the battery modules and configured to wirelessly transmit information including the current value and the voltage value synchronous with each other, the current value detected by the current detection unit, and the voltage value detected by the voltage detection unit; a master unit configured to receive the information wirelessly transmitted from the of slave units; and a central monitoring unit configured to (i) receive the information received wirelessly by the master unit, (ii) calculate an internal resistance of each cell from the information, and (iii) further calculate a state of charge of each cell based on the calculated internal resistance of the cell.
 2. The battery monitoring system according to claim 1, wherein the at least one current detection unit comprises a single current detection unit for the slave units, and each slave unit acquires information on the current value from the single current detection unit.
 3. The battery monitoring system according to claim 1, wherein the at least one current detection unit comprises a plurality of current detection units each provided for a corresponding one of the slave units, and each slave unit acquires information on the current value from a corresponding one of the current detection units.
 4. The battery monitoring system according to claim 1, wherein the current detection unit outputs information on the current value as a digital signal, and the battery monitoring system comprises isolators on a path carrying the digital signal, the isolators configured to transfer the digital signal while providing isolation.
 5. The battery monitoring system according to claim 1, wherein the voltage detection unit acquires information on the current value from the current detection unit, and the slave unit acquires information on the voltage value and the current value from the voltage detection unit.
 6. The battery monitoring system according to claim 1, wherein the slave unit acquires information on the voltage value from the voltage detection unit, and the slave unit acquires information on the current value from the current detection unit, not through the voltage detection unit.
 7. The battery monitoring system according to claim 1, wherein each slave unit wirelessly transmits information on the current value more frequently than information on the voltage value.
 8. The battery monitoring system according to claim 1, wherein the battery monitoring system has no path for carrying information on the current value via cable from the current detection unit to the central monitoring unit, and each slave unit wirelessly transmits the information on the current value at different times from the other slave units.
 9. The battery monitoring system according to claim 1, wherein the current detection unit includes a shunt resistor connected in series to the battery modules, and the current detection unit also includes a calculator configured to calculate the current value from a voltage difference across the shunt resistor.
 10. The battery monitoring system according to claim 1, wherein the current detection unit includes (i) a capacitor charged by being connected to a corresponding one of the battery modules, thereafter the capacitor being disconnected, and (ii) a calculator configured to calculate the current value from a duration of the capacitor being charged and a voltage of the charged capacitor. 